Rate-adaptive cardiac pacemakers are known which control the adaptive stimulation rate by the use of a comparison between the current sensor signal morphology and the morphology determined in the resting patient. Thus, for example, in the case of the "Inos" pacemaker of the applicant the respective current intracardial impedance curve is compared with a reference curve and the adaptive rate is determined from the integral difference. In the case of such pacemakers, it is important to keep the reference curve constantly current on the one hand, but on the other hand to only actually update it in the resting patient. For such an automatic updating of the reference curve, detection of the physical positions (standing or sitting or lying down) of the patient is desirable, since, within certain limits, the position detection also enables the detection of resting phases of the patient.
With rate-adaptive cardiac pacemakers, an additional problem may exist in that the signal morphology of a measured signal used for the rate adaptation exhibits great changes when the patient changes positions without the cause being capable of detection by a rate adaptation algorithm. This can result in paradoxical pacemaker behavior, such as a counterproductive drop in the stimulation rate when the patient stands up. A means of independent position detection would be of great utility, so that the rate adaptation algorithm could receive an additional input signal in order to be able to react properly to changes in position.
Of no less importance, is a determination of the proper time for an automatic night reduction in the stimulation rate by setting the time basis of the pacemaker independent of the influence of time zone changes or changing daily rhythm of the patient using the patient's actual lying down and resting phases; cf. PCT International Publication No. W091/08017.
Proposals have long been known to detect the physical position of patients using mercury or similar position-sensitive switches. However, because of various problems, to date, these have not proven to be effective in practice. In basic research, the heart rate variability in predefined low frequency (LF) intervals (0.05-0.15 Hz) and high frequency (HF) intervals (0.15-0.4 Hz) is used as an indicator of sympathovagal equilibrium which is affected by a change in position; cf. S. Akselrod et al. in "Hemodynamic Regulation: Investigation by Spectral Analysis", Am. J. Physiol., 249, H 867-875 (1985). The process is not applicable in the particularly important and practical area of pacemaker patients since the variability of the heart rate in pacemaker patients is usually not available.